In the post-war Constitution of Japan, the Emperor has only ceremonial powers. As such, Hirohito played no direct role in the rebuilding of Japan's economy.

I suppose one can argue that the Emperor, by the fact of his existence, played an indirect role since he was able to be "used" by the occupational government to ensure some degree of continuity during the immediate post-war period. Japan's relatively peaceful occupation no doubt helped the economic recovery, and having the Emperor around helped to ensure that peace. However, this is a rather indirect role, and Japan's main economic boom occurred after the end of the Allied occupation.